As the supply of portable small electrical and electronic devices is expanding, new secondary batteries such as a nickel metal hydride battery, a lithium secondary battery, a super capacitor, and a lithium ion capacitor have been actively developed.
Among them, the lithium ion capacitor (LIC) is a new concept of the secondary battery system, which combines the high output/long-life characteristics of conventional electric double layer capacitors (EDLC) with the high energy density characteristic of lithium ion batteries.
The electric double layer capacitor utilizing physical adsorption of charges in the electric double layer has excellent output and life span characteristics, but its low energy density limits its use in many applications. To solve this problem of the electric double layer capacitor, a hybrid capacitor has been suggested, in which energy density is improved by using a material capable of intercalating and extracting lithium ions as a cathode or anode active material. In particular, the lithium ion capacitor has been suggested, in which a material used in the conventional electric double layer capacitors is used as a cathode material and a carbon-based material capable of intercalating and extracting lithium ions is used as an anode active material.
For instance, as shown in FIG. 1, the electric double layer capacitor shows excellent output characteristic, based on adsorption and desorption of charges by symmetrical use of an activated carbon material having a high specific surface area for both cathode and anode, but it has a drawback of low energy density (Ea). In contrast, the hybrid capacitor has the characteristic of high capacitance (Eb) by utilizing high-capacitance transition metal oxide as the cathode material, and the lithium ion capacitor has the characteristic of improved energy density (Ed) by utilizing the carbon-based material capable of reversibly intercalating and extracting lithium ions as the anode material.
Of them, since the lithium ion capacitor utilizes a material capable of intercalating and extracting lithium ions at a low reaction potential as the anode active material, it shows much higher improvement in energy density than other hybrid capacitors. In the lithium ion capacitor, lithium ions having high ionization tendency is pre-doped on the anode to greatly reduce the electrical potential of the anode. Further, the lithium ion capacitor has a cell voltage of 3.8 V which is much higher than 2.5 V of conventional electric double layer capacitors, and higher energy density.
With regard to the reaction mechanism of the lithium ion capacitor which includes an anode consisting of a carbon-based material doped with lithium ions, electrons are transported to the carbon-based material of the anode and thus the carbon-based material is negatively charged, leading to intercalation of lithium ions into the carbon-based material of the anode, at the time of charging. Meanwhile, lithium ions intercalated into the carbon-based material of the anode are extracted and the negative ions are adsorbed on the cathode, at the time of discharging. Doping amount of the anode with lithium ions can be controlled by this reaction mechanism, thereby providing a lithium ion capacitor having a high energy density. Further, the lithium ion capacitor is a system that combines the good energy storage of lithium ion batteries with the high output characteristic of capacitors, and is a futuristic battery system that exhibits the capacitor characteristics at a high output power and the life span as long as that of lithium ion batteries by utilizing a material having both of the functions.
However, the lithium ion capacitor requires electrochemical adsorption/desorption as well as a lithium doping process for lithium intercalation and extraction. In the conventional technology of doping the anode with lithium for the lithium ion capacitor, metal lithium is laminated onto an electrode, and then the negative electrode and metal lithium are short-circuited by injection of an electrolytic solution, and at this time, an electrical potential difference occurs between the negative electrode and the lithium metal, so that it is possible to naturally dope the lithium metal into the negative electrode. The lithium doping method carried out by laminating metal lithium onto the electrode and short-circuiting them, however, has problems that it is difficult to control the doping amount of lithium on the negative electrode and safe handling of lithium metal should be secured during the doping process, and thus the method is not suitable for large scale use.
Accordingly, there is a need to develop a material for lithium ion capacitor, which exhibits excellent capacitor characteristics of high output power, long life span and high energy density and has excellent safety to be suitable for large scale use, and a production process thereof.